Ublox SARA-G450 Series - Page 111

SARA-G450 - System integration manual
UBX-18046432 - R08 Design-in Page 111 of 143
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2.7.1.3 Guidelines for analog audio layout design
Accurate analog audio design is very important to obtain clear and high quality audio. The GSM signal
burst has a repetition of 217 Hz that lies in the audible range. A careful layout is required to reduce the
risk of noise from audio lines due to both VCC burst noise coupling and RF detection.
Guidelines for the uplink path, which is the most sensitive since the analog input signals are in the
microvolt range, are as follow below:
 Avoid coupling of any noisy signal to the microphone lines: it is strongly recommended to route
microphone lines away from module VCC supply line, any switching regulator line, RF antenna
lines, digital lines and any other possible noise source.
 Avoid coupling between microphone and speaker / receiver lines.
 Optimize the mechanical design of the application device, the position, orientation and mechanical
fixing (for example, using rubber gaskets) of microphone and speaker parts in order to avoid echo
interference between the uplink path and downlink path.
 Keep ground separation from microphone lines to other noisy signals. Use an intermediate ground
layer or vias wall for coplanar signals.
 Route microphone signal lines as a differential pair embedded in ground to reduce differential
noise pick-up. The balanced configuration will help reject the common mode noise.
 Route microphone reference as a signal line since the MIC_GND pin is internally connected to
ground as a sense line as the reference for the analog audio input.
 Cross other signals lines on adjacent layers with 90° crossing.
 Place the bypass capacitors for RF very close to the active microphone. The preferred microphone
should be designed for GSM applications which typically have internal built-in bypass capacitor for
RF very close to the active device. If the integrated FET detects the RF burst, the resulting DC level
will be in the pass-band of the audio circuitry and cannot be filtered by any other device.
Guidelines for the downlink path are the following:
 The physical width of the audio output lines on the application board must be wide enough to
minimize series resistance since the lines are connected to a low impedance speaker transducer.
 Avoid coupling of any noisy signal to speaker lines: it is recommended to route speaker lines away
from the module VCC supply line, any switching regulator line, RF antenna lines, digital lines and
any other possible noise source.
 Avoid coupling between speaker / receiver and microphone lines.
 Optimize the mechanical design of the application device, the position, orientation and mechanical
fixing (for example, using rubber gaskets) of speaker and microphone parts in order to avoid echo
interference between the downlink path and uplink path.
 Route speaker signal lines as a differential pair embedded in ground up to reduce differential noise
pick-up. The balanced configuration will help reject the common mode noise.
 Cross other signals lines on adjacent layers with 90° crossing.
 Place the bypass capacitors for RF close to the speaker.